Fume hoods have been used in laboratories and other places for many years and, in general, employ a bench structure having a work surface upon which is disposed an enclosing hood structure, the front side wall of which is formed, at least in part, by an upwardly movable sash member, usually having a glass panel through which the interior of the hood may be viewed, and which member may be disposed in a closed position or may be suitably raised to an open position to provide a desired amount of access to the interior. In most instances, the sash member is counterbalanced for ease of operation, and at the same time retaining the sash member in any desired open position.
In use, air is exhausted from the hood enclosure by means of a suitable exhaust blower or the like to withdraw obnoxious or dangerous gases, etc. created within the hood, with air being admitted through the access opening to maintain a control velocity through the open face of the hood structure to prevent any contaminated air from escaping into the room in which the hood is disposed. In some cases the entire air withdrawn may be supplied through the access opening from the room air, while in other installations an auxiliary air supply and air blower are provided for supplying air to the hood and thus reduce the total volume of air withdrawn from the room. For example, assuming an exhaust of 1,000 CFM of air, with a 70% auxiliary air ratio, 700 CFM would be supplied by the auxiliary air supply and only 300 CFM by the room.
Where a single speed exhaust blower, with or without an auxiliary air blower is employed a relatively large volume of air is being continually exhausted, with all or at least a part thereof being withdrawn from the room interior. If a number of hoods are in a room the loss is accordingly multiplied. Where such room interior is either heated or air conditioned, obviously the loss of a large volume of room air entails a corresponding loss in energy involved in the supply of such air, and if the volume of air could be materially decreased when not required, a considerable saving in operating costs could be achieved.